Twenty-seven miles long and brimming with energy, PG&E's proposed transmission line is poised to power San Francisco and the Peninsula for years.

Or, if you believe some concerned residents and local officials who have protested the proposal, it's a coiled snake that poses a serious health threat.

On Thursday, the California Public Utilities Commission will vote on an alignment for Pacific Gas and Electric Co.'s $207 million Jefferson-Martin Transmission project. The line will travel north from the Jefferson Substation west of Redwood City to the Martin Substation.

The controversial 230,000-volt line, a combination of underground and overhead wires, will create a link between substations in Brisbane and Redwood City, adding electrical capacity.

PG&E is touting the project as a way to meet rising demand and to provide a hedge against blackouts.

"As time goes on in all communities in the United States, demand for electricity continues to grow," said PG&E spokesman Paul Moreno. "The growth sometimes slows or increases, but we have to build infrastructure to meet that. "

On Thursday, the PUC will select an alignment from two proposals that are nearly identical -- except that one explores the possibility of a detour over San Bruno Mountain, away from several Daly City schools.

The project, six years in the making, has raised questions about the danger the line might pose to residents. Critics are concerned by the electromagnetic fields (EMFs) created by the lines, part of an evolving area of scientific research that has yet to reach consensus on the potential health threats to humans.

According to a 2002 scientific review by the California Department of Health Services, which surveyed published studies, these fields might cause increased risk of childhood leukemia, adult brain cancer, Lou Gehrig's disease and miscarriage. However, scientists said there is still no conclusive link tying EMFs to these diseases.

PG&E officials have pointed out the lack of a causal relationship, but residents said there is enough evidence to warrant caution.

"There are a lot of people here who are very concerned about EMFs," said Katie Carlin, an organizer with the 280 Corridor Concerned Citizens. "This issue is not going away. New studies are being released, and scientists are redoing their old studies and finding not only associations but (eventually) causations."

Carlin's organization represents several hundred households along Skyline Boulevard in Burlingame, Hillsborough and unincorporated San Mateo County, where the line is scheduled to travel underground near homes.

The citizens group has proposed running the line a mile to the west -- through the San Francisco Peninsula Watershed -- and although its plan is not being considered by the PUC, residents have continued to lobby individual commission members.

On nearby Trousdale Drive in Burlingame, residents also protested an early draft of the plan that had the line running along their busy road. On June 9, however, a PUC administrative law judge noted the potential EMF threat to Trousdale residents and ruled out that alignment.

But residents there said the issue still needs to be dealt with along the entire length of the project. Dennis Zell, co-chair of Concerned Residents of Burlingame, who fought the Trousdale alignment, said the commission should also heed administrative law judge Charlotte TerKeurst's suggestion to update its EMF studies, its maximum exposure standards for EMF in residential areas and its mitigation measures.

"We feel like we dodged a bullet," Zell said. "But the broader question is: When is the PUC going to recognize the danger EMF poses to children and pregnant women, and when are they going to change their rules?"

PG&E's Moreno said the people are exposed to EMFs from television sets, computers, lights and other electronic equipment. He said PG&E also will be spending millions of dollars to minimize the threat of EMFs.

"The fact is we live with EMFs every day," Moreno said. "If these people were really concerned about EMFs, they'd turn off their electricity in their homes and homeschool their kids. It's like complaining about secondhand smoke while puffing on a cigarette."

But critics are still not buying it. Daly City Mayor Sal Torres, for one, said the issue is too uncertain and fraught with hidden dangers to plod ahead blindly. He is pushing for a detour over San Bruno Mountain that will keep the line away from three elementary schools and one middle school.

While PG&E said the wires would not be close enough to register any EMFs at three of the four schools during median electrical loads, Torres said it's too risky.

"There are enough question marks for me," he said. "I can tell you with absolute certainty that running the wires over (San Bruno Mountain) will not be near these children and adults, and that is a heck of a lot better than running them under humans and taking a chance. It's like rolling a dice and hoping that in 20 years we'll have a good roll."

But Torres' plan has angered environmentalists, who say it would do serious damage to endangered animals and plant species.

"We're talking about adding huge towers and significant ground disturbance and, frankly, PG&E has a miserable record of mitigating for damage to habitats," said Philip Batchelder, program director for the San Bruno Mountain Watch.